Light-triggered Time-Programmed Self-adhesion in Low-Hysteresis Hydrogels

Self-healing of hydrogels has attracted intensive research based on dynamically exchangeable bonds. However, this typically leads to decreased elasticity and increased energy dissipation during mechanical deformations, which is not desirable in, for example, soft robotic applications. Thus, there exists an unmet demand for low hysteresis, that is, resilient, materials to quickly recover the mechanical properties after damage. Here, we show low-hysteresis chemically cross-linked hydrogels with on-demand local light-triggered fast self-adhesion, with time-dependent adhesion strength controlled by the duration of irradiation. Low mechanical hysteresis is achieved by swelling of the loosely cross-linked poly(N-isopropylacrylamide) network. Rapid self-adhesion is followed by localized photothermal heating of embedded gold nanoparticles, causing collapse and promoting local entanglements of the thermoresponsive poly(N-isopropylacrylamide) chains. This provides 95.7% initial recovery of the original mechanical properties, while the interface undergoes gradual rehydration and disentanglement depending on the irradiation details, leading to a decrease of the adhesive strength to zero within 7 h. The concept can be combined with conventional slow self-healing that is allowed by additional clay nanoplatelets for long-standing healing. The application potential is demonstrated by oscillations, time-programmed release of the adhered object, and on-demand assembly of designed hydrogel shapes. The proposed mechanism with facile, efficient, and light-controlled temporal profiles that are time-programmed can be applied to soft robotics, biomedical applications, and flexible electronics.